1. Field of the Invention
The present invention relates to novel polymers capable of absorbing large quantities of water or aqueous liquids, and, more especially, to novel superabsorbent acrylic polymers and the preparation thereof via inverse suspension polymerization.
2. Description of the Prior Art
Superabsorbent polymers are currently widely used for the manufacture of sanitary products designed to absorb and retain body fluids.
The production of superabsorbent polyacrylate powders via inverse suspension polymerization is described in French Patent No. 2,367,083 (Kao Soap). This process entails polymerizing an acrylate in aqueous suspension (preliminarily suspended in an aliphatic solvent by means of an emulsifying agent having an HLB of from 3 to 6) and in the absence of a crosslinking agent. The properties of the resultant polymer powders are improved if an emulsifying agent having an HLB (hydrophilic/lipophilic balance) of from 8 to 12 is used (EP-36,463, Seitsu Kagaku Co., Ltd.). The most apparent difference between these two techniques is in the morphology of the polymer powders produced, the first being in the form of spheres and the second in the form of particles having more uneven shapes.
One major improvement is provided by carrying out the process in two stages, as described in European Patent No. EP-0,441,507. Thus, in a first step, a gel is formed via polymerization of an acrylic solution in inverse suspension, next absorbing a second monomer filler into the gel, and then effecting the polymerization of said second monomer within the gel itself.
Nonetheless, the aforesaid processes are not suited for the preparation of superabsorbent polymer powders which satisfy the requirements of certain major sectors of the industry, since the absorbent powders thus obtained have an excessively long gelling or gelation time, or because, even though they indeed exhibit an acceptable gelling time, this property is attained at the expense of the other physical properties of the product, or of their contents in extractable materials.